Manatees feed on aquatic vegetation at the bottom, the surface, and along the banks. They are generalist grazers, meaning that they are not restricted to a specific food source. Thus, while seagrasses are an important component of the manatee diet, they will also consume a wide variety of aquatic vegetation, even reaching out of the water to graze on overhanging leaves or grasses. The following is a general description of seagrass beds, which are critical for manatee survival in Florida.
Figure. Manatee feeding on floating seagrass and
mangrove leaves (photo: USGS – Sirenia Project).
Figure. Manatee feeding on seagrass
(photo: USGS – Sirenia Project).
Seagrass Beds in Florida
Seagrasses are marine plants that are found in the soft intertidal sediments near shorelines. The plants are specially adapted to their aquatic environment. They occur worldwide and include between 50 to 60 species in two grass families, Hydrocharitaceae and Potamagetonaceae (Williams & Heck Jr., 2001). In Florida, a few species are found regularly in coastal areas (Dawes, 1987). These include Thalassia testudinum (Turtle Grass), Syringodium filiforme (Manatee Grass), and Halodule wrightii (Shoal Grass). Vallisneria americana (Eel Grass) and Ruppia maritime (Ditch Grass) are also commonly found in the waters of southwest Florida. Seagrasses are clonal flowering plants that can form extensive beds that may be dominated by one species of grass, but which usually include several species. Seagrass plants have well-developed below ground mats of rhizomes and fibrous roots. Above ground, seagrass leaves are paddle-like or strap-like in shape. Seagrass beds may be large and extensive, covering many square kilometers.
Biological Diversity of Seagrass Beds
Seagrass beds are highly productive biological systems, producing as much as some intensive terrestrial agricultural systems (McRoy & McMillan, 1977). The richness and productivity of seagrass habitat has been well-documented since the early 1920s (Dawes et al., 2004). Dawes et al. defined seagrass habitat as “a physical space containing seagrasses in sufficient quantity and pattern to produce the appropriate structural and physiological characteristics to support organisms typical of seagrass communities.” The diversity of animals and plants associated with seagrass beds is impressive. Dozens of species of algae live as epiphytes on the leaf blades and within the root mass of the seagrass beds. A plethora of small marine animals, such as larval fish and crustaceans, feed on seagrass detritus, algae, and other tiny marine animals that use seagrass beds for cover and food. Larger predatory and carnivorous fish are attracted to seagrass beds for the supply of prey they harbor. Grazing marine animals, including sea turtles and manatees, feed on seagrasses. Wading birds frequent seagrass beds and raptors hunt over them (Williams & Heck Jr., 2001).
Ecological Function of Seagrass Beds
Seagrass ecosystems fulfill vital ecological functions in the maintenance of estuaries and coastal marine environments. Their structure affects the flow of water locally, dampening the effects of waves and thereby altering erosion and sedimentation rates, nutrient and microorganism fluxes, and recruitment of larval stages of marine animals. Seagrass beds provide refuge from predators for small fish and crustaceans, and act as nurseries for some species (Dawes, 1987; Dawes et al., 2004; Williams & Heck Jr., 2001). They take up vast amounts of carbon (Zieman & Wetzel, 1990), acting as a “carbon sink,” a valuable function considering the problems of increasing atmospheric CO2 and its effects on global climate change.
Threats to Seagrass Beds
Threats to seagrass beds include eutrophication, changes in water salinity, and damage from boat propellers. Eutrophication has been linked to the effects of coastal development, including higher levels of sediment and nutrients in storm water runoff. These in turn raise water temperatures and attenuate light, stimulating the production of algae. Increased algae reduce light availability and dissolved oxygen in the water column and result in lower seagrass production rates.
Figure. Scarred seagrass bed.
Natural variations in water salinity influence the distribution and species composition of seagrass beds. Some seagrasses are found in more saline, marine environments, while others are found in estuarine salinity ranges, and yet others in brackish and freshwater. When water salinity concentrations change, the species composition and geographic distribution of the seagrass beds change. For example in the case of the Caloosahatchee River, the release of freshwater from the Franklin Lock and Dam structure may have affected the distribution of seagrass beds. Studies conducted between 1986 and 1997 sampled seagrass beds in the Caloosahatchee River. However, during a 1999 study, researchers were unable to find seagrass beds in the lower reaches of the river (Chamberlain & Doering, 1998; Florida Fish and Wildlife Conservation Commission, 2002).
Physical damage to seagrass beds is a concern where boat propellers leave scars in seagrass beds that take years to recover. Scars occur “when a boat’s propeller tears and cuts up roots, stems, and leaves of seagrasses, producing a long narrow furrow devoid of seagrass” (Sargent et al., 1995). In Florida, scarring is a serious problem affecting the productivity of seagrass beds. The extent of the damage caused by propellers is regularly monitored by resource managers in coastal areas.